Is there a way to export a non-square UV map?
Imagine the square of the UV space as percentage of the texture. Its 100% of its width across the width of the square and 100% of the height of the image across the height of the square. It doesn't matter what the aspect ratio is, the texture will fill the square. If you fill the square with the UVs of a rectangular mesh and then use a rectangular image matching the aspect ratio of the mesh, it will work.

If you open an image in the UV editor, the UVs will be displayed(and transformed) on that image according to the aspect ratio of it. That might be more intuitive to work with.